Hammer is a SDK that is developed by Valve. It is commonly used for editing maps in various source engine games including Team Fortress 2, Counter-Strike, DOTA 2, Portal 2 and more.

Issues

Hammer compiles but won't run map

Hammer successfully compiles a map when you click on Run Map from the File menu but does not launch the game.

Solution

To fix this, simply change the options when you are compiling.

Only VisGroup is available when you click properties

Only the VisGroup tab is shown when you try to load the properties of an entity or brush.

Solution

In the top toolbar, click on the IG button or Toggle group ignore.